build: create the correct $GOTOOLDIR.
build: create the correct $GOTOOLDIR.

Before this, make.bash --dist-tool would fail,
because $GOROOT/bin/tool was being mkdir'd
instead of $GOROOT/pkg/tool/linux_amd64.

R=rsc
CC=golang-dev
http://codereview.appspot.com/5666046
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go